1. Field of the Invention
The present invention relates to a method of forming an orientation film of a liquid-crystal display device and more particularly to a method for forming a film consisting of liquid-crystal molecules oriented in a horizontal and unidirectional manner.
2. Description of the Related Art
A normal liquid-crystal display device is designed to provide liquid crystal between transparent electrodes consisting of glass plates opposite to each other. By applying an electric signal between these electrodes, the display device is capable of modulating an incident ray from the outside and displaying information. For obtaining a feasible contrast, it is necessary to uniformly control the orientation of liquid-crystal molecules. In particular, for a liquid-crystal display device employing a field-effect system such as a nematic system, which is designed to twist the liquid-crystal molecules by about 90.degree. to 360.degree. between the upper and the lower cells, it is preferable to orient the liquidcrystal molecules in a horizontal and unidirectional manner in light of the principle of operation.
As a method for forming a film consisting of liquidcrystal molecules oriented in a horizontal and unidirectional manner, in general, there has been employed a rubbing method or an oblique evaporation method. The former method includes the steps of forming on an electrode plate an orientation film consisting of silicon oxide or polyimide, for example, applying sintering treatment on the electrode plate having the film formed thereon if necessary, and abrading the electrode plate in a unidirectional manner with a gigged or flocked cloth. The latter method includes the steps of vacuumvaporizing silicon oxide on the electrode plate in an oblique manner and growing the vaporized silicon oxide to be a vaporized film in a specific direction. These methods can offer great restriction for orientation to the liquid-crystal molecules so that they allow the liquid-crystal molecules to be stably oriented for a long period. Hence, they are widely employed as a method for treating orientation of the liquidcrystal molecules. In addition to the foregoing methods, there has been employed another method including the steps of forming a film consisting of silicon oxide on an electrode plate and radiating grains like ions in an electrically accelerating manner for making the silicon-oxide film served as the orientation film.
The foregoing rubbing method and oblique vaporization method, however, have the following shortcomings.
According to the rubbing method, the rubbing pressure is unevenly applied to a large electrode plate because the plate has an uneven surface and a heterogeneous thickness. The unevenly applied pressure results in unevenly orienting the liquid-crystal molecules, that is, being unable to realize high-precision display. And, as the electrode plate is abraded with the rubbing cloth, fibers and impurities contained in the cloth are attached on the orientation film, thereby degrading the characteristic of the display. It is thus indispensable to clean the liquid-crystal display, resulting in necessitating a large quantity of cleaning solvent. Moreover, in the liquid-crystal display containing a nonlinear element array such as a TFT (thin film transistor) or a MIM (metal-insulating layer-metal) element, the static electricity caused when abrading the treated electrode plate may break down a semiconductor switching element.
According to the oblique evaporation method, the crystals of silicon oxide attached on the electrode plate are expanded in a fanlike manner. It results in disadvantageously being unable to restrict the orientation direction of the liquid-crystal molecules to be unidirectional as well as unevenly orienting the molecules because of the flakes caused by the fanlike expansion of the crystals.
Lastly, consider the foregoing third method, that is, an orientation film forming method designed to radiate grains such as ions on the electrode plate having a silicon oxide film formed thereon in an electrically accelerating manner. FIG. 1 shows the characteristics of an orientation film formed to have a thickness of 1000.ANG. with a normal sputtering method. As shown in FIG. 1, this method offers incomplete orientation of a film, so that it cannot realize characteristics sufficiently meeting the operation of the liquid-crystal display device.